As shown in Patent Document 1, there is a known screw expander system in which a generator is driven by an oil-feed type screw expander (screw turbine). As shown in FIG. 6, this screw expander system 100 includes a generator main body 105 in which a rotor shaft 103 of a screw rotor 102 of a screw expander 101 is coupled to a generator 104, an oil separating tank 106, a condenser 107, an operating medium pump 108, an evaporator 109, an oil service tank 110, an oil tank 111, and an oil pump 112.
The screw expander 101 expands an operating medium sent through an operating medium inlet 113 by the screw rotor 102, and discharges from an operating medium outlet 114. In the screw expander 101, oil is supplied from an oil inlet 115 to a bearing 116 supporting the rotor shaft 103, the oil is discharged from the operating medium outlet 114 communicating with an oil outlet 117, the oil is supplied from an oil inlet 120 communicating with the operating medium inlet 113 to an outer circumferential surface of the screw rotor 102, and the oil is discharged from the operating medium outlet 114. The operating medium and the oil discharged from the operating medium outlet 114 become a mixture, which is sent to the oil separating tank 106. In the oil separating tank 106, the mixture is separated into the oil and the operating medium. The operating medium separated in the oil separating tank 106 is liquefied in the condenser 107, and sent to the evaporator 109 by the operating medium pump 108. The operating medium is evaporated by the evaporator 109, and sent to the operating medium inlet 113 of the screw expander 101. The oil is sent from an oil discharge port 118 of the oil separating tank 106 to the oil tank 111. The oil is sent by the oil pump 112 from the oil tank 111 to the bearing 116 and the outer circumferential surface of the screw rotor 102 through the oil inlets 115, 120 of the screw expander 101. In the screw expander system 100, the oil service tank 110 is disposed in an oil supply line 119 connected so as to be branched from the upstream side of an inlet of the oil separating tank 106 and merged with the downstream side of the oil discharge port 118, and the oil is supplemented from the oil service tank 110 to the oil tank 111 in accordance with a pressure difference between the upstream side of the inlet of the oil separating tank 106 and the downstream side of the oil discharge port 118.
However, since the oil outlet 117 of the screw expander 101 has a low pressure in comparison to the oil inlets 115, 120, the oil pump 112 is required for supplying the oil from the oil tank 111 on the downstream of the oil outlet 117 to the screw expander 101. Size of the oil separating tank 106 depends on flow speed of a gas or a volume flow rate (for example, substantially proportional to the flow speed of the gas or the volume flow rate). As described above, the operating medium outlet 114 of the screw expander 101 has a low pressure in comparison to the operating medium inlet 113, and the volume flow rate of the gas flowing into the oil separating tank 106 is increased. Thus, there is a need for increasing the size of the oil separating tank 106, and there is a problem that size of the screw expander system 100 is increased.